The present invention relates generally to a method and apparatus for material processing, and more particularly, to a method and apparatus for sintering metal parts.
In metal processing, various thermal treatment operations such as annealing, hardening, brazing and sintering are often performed under inert or reducing atmospheres in order to avoid and/or remove oxidation from metal parts. In powder metallurgy, for example, high pressure is applied to metal powders to form compacts which are then sintered in a furnace to form metal parts. Sintering of the compacts is typically performed under an inert or reducing atmosphere such as a mixture of nitrogen (N2) and hydrogen (H2) or an Endo gas mixture containing carbon monoxide (CO), H2 and N2. The sintered metal parts are then subjected to a cooling phase, during which transformation of the microstructure of the metal parts may occur. Certain metal parts may also be subjected to sinterhardening, i.e., transformation to a hard martensite phase during cooling. Sinterhardening is typically carried out in a cooler such as a convection cooler, with alloy additives such as nickel, molybdenum, among others, added to the metal powders prior to sintering. These alloy additives are used to facilitate sinterhardening of the metal parts, resulting in products that are either harder or tougher than non-sinterhardened parts. Water coolers, which provide slower cooling than convection coolers, may also be used with more expensive types of powder mix to provide metal parts with increased martensite phase.
Much of the efforts for improving sintering methods have focused on the control of process conditions during sintering. However, since the transformation of microstructure during the cooling phase directly affects the material properties of the processed parts, there is a need for an improved method of sintering by controlling process conditions during the cooling phase.
The present invention provides generally a method and an apparatus for metal processing. According to one aspect of the invention, a method is provided for sintering a workpiece in a heating zone of a furnace, and exposing the workpiece in a cooling atmosphere in a cooling zone of the furnace. In one embodiment, the cooling atmosphere contains at least about 15% of hydrogen, at least a portion of which is introduced via an inlet within the cooling zone.
In another embodiment of the invention, a method provides for sintering a powder metal part in a heating zone of a furnace, and exposing the powder metal part to a cooling atmosphere in a cooling zone of the furnace. The cooling atmosphere comprises a first gas at a concentration of at least about 25% and a second gas at a concentration of at least about 5%. The first gas is selected from hydrogen, helium and combinations thereof, while the second gas is selected from nitrogen, argon and combinations thereof. At least a portion of the first gas is introduced into the cooling zone via an inlet in the cooling zone.
In yet another embodiment, a method provides for sintering a workpiece in a heating zone of a furnace, introducing hydrogen to form at least a part of a cooing atmosphere in a cooling zone adjoining the heating zone, and exposing the workpiece to the cooling atmosphere. The hydrogen is introduced via an inlet that is configured such that the hydrogen reaches the cooling zone prior to the heating zone, and the cooling atmosphere contains sufficient hydrogen to provide a cooling rate for the workpiece that is at least about 30% higher than a cooling rate obtained in a cooling atmosphere containing no hydrogen.
In yet another embodiment, a method provides sintering a workpiece in a heating zone of a furnace, and cooling the workpiece in the cooling zone by exposing it to a cooling atmosphere containing hydrogen. At least a portion of the hydrogen is introduced via an inlet within the cooling zone, and the cooling atmosphere has a hydrogen concentration that is at least about 10% higher than a hydrogen concentration in the heating zone.
Another aspect of the present invention provides for a sintering furnace comprising a heating zone, a cooling zone with a gas inlet for introducing a cooling feedgas comprising hydrogen, and a means for inhibiting gas flow between the cooling zone and the heating zone.